Method of preparing lubricative crystalline molybdenum disulfide



United States Patent METHOD OF PREPARING LUBRICATIVE CRYS- TALLINE MOLYBDENUM DISULFIDE Giinter Spengler and Hannsheinz Hohn, Munich, Germany, assignors to The Alpha Molykote Corporation, Stamford, Conn.

No Drawing. Application August 29, 1955 Serial No. 531,301

Claims priority, application Germany September 1, 1954 4 Claims. (Cl. 148-624) This invention relates to a novel and improved method of preparing molybdenum disulfide in the form of crystals having superior lubricating properties and to lubricants containing molybdenum disulfide. In accordance with one preferred embodiment of the invention, a structural member initially having metallic molybdenum or a compound of molybdenum in or on a surface portion thereof is sulfided in situ to convert the treated surface into a molybdenum disulfide-containing bearing surface a of superior lubricity.

\ It is well known that natural molybdenum disulfide extracted from molybdenite ore and separated from clay and other impurities has excellent lubricating properties. It is believed that the lubricity of molybdenum disulfide is due at least in part to its unique laminar crystal structure in which a lamina of molybdenum atoms is in effect sandwiched between two laminae of sulfur atoms.

While some attempts have been made heretofore to prepare molybdenum disulfide synthetically from metallic molybdenum, molybdenum oxides and salts of molybdic acid, the methods proposed have been generally un satisfactory because they involved long heating to temperatures well above 500 C. and in some instances over 1000 C. Such high temperatures are not only undesirable from an economic and technical point of view but discourage attempts to form molybdenum disulfide in situ on a bearing surface because of the distortion, thermal stress and heat treatment efiects on the structural bearing members themselves.

The surprising discovery has now been made that lubricative crystalline molybdenum disulfide can be prepared in powder form as well as in situ on a bearing surface at substantially lower temperatures and in shorter periods of time by sulfiding a molybdenum-containing material in an alkaline medium.

In accordance with the invention, the sulfiding agent used is elemental sulfur or a compound containing sulfur in divalent form. Sodium and potassium polysulfides which, by themselves or in the form of a eutectic mixture, have a melting point of less than about 400 C. are particularly desirable. Hydrogen sulfide may also be used to advantage. The molybdenum-containing material may be in its elemental state or in a physically combined state, e.g., free or alloyed metallic molybdenum or in the form of a compound such as molybdenum dioxide, molybdenum trioxide, precipitated or amorphom molybdenum dior trisulfide, a salt of molybdic acid, an isopolymolybdate, a molybdenum halide or a mixture of such compounds. The alkaline medium is most desirably selected from the group consisting of alkali-, alkaline earthand ammonium hydroxides, sulfides and carbonates as well as mixtures thereof, sodium and potassium hydroxides and carbonates being preferred.

In one preferred embodiment of the method of this invention molybdenum-containing material, a term used herein to refer to molybdenum in its elemental as well as in its chemically and physically combined states, is heated to an elevated temperature of the order of about 2,892,741 Patented June 30, 1959 300 to about 500 C. with a stoichiometric excess of sulfur or hydrogen sulfide in the presence of an alkaline medium or, still more preferably, with an alkali metal polysulfide or mixture of such polysulfides containing a stoichiometric excess of sulfur until the molybdenum has been sulfided. The use of alkali metal polysulfides in the method of the invention is preferred because it furnishes both the alkalinity and the sulfur required for the sulfiding of the molybdenum.

Generally, the sulfiding is completed in between about one-half hour and twelve hours at temperatures below about 500 C. The sulfiding agent and alkaline medium or, if these two are in effect combined, the alkali metal polysulfide, need not necessarily form a melt at temperatures below 500 C. if they have previously been mixed intimately with the molybdenum. Generally, a sintered cake is produced which contains substantially all of the molybdenum disulfide in insoluble form. If temperatures above about 500 C. are utilized, the molybdenum sulfides tend to form thiomolybdates accompanied by evaporation of excess sulfur. On the other hand, at temperatures below about 350 C. the yield of molybdenum sulfides falls off fairly rapidly.

In carrying out the method of the invention, particularly at above about 400 C., it is advantageous to conduct the sulfiding operation in a closed vessel and under superatmospheric pressure generated thereby. It is also advantageous to conduct the sulfidization in a non-oxidizing and inert atmosphere. The sulfiding agent should advantageously be present in amounts ranging from about 1.1 to about four times the stoichiometric amount re quired for conversion of the molybdenum into M08 Lesser amounts of sulfur in the sulfiding agent or agents are to be avoided as wasteful of molybdenum and greater amounts, while operable, serve no useful purpose.

One of the most outstanding advantages of the method of the invention is its adaptability to producing firmly adhering molybdenum disulfides in situ on the surfaces of metallic, ceramic and similar objects. This is accomplished by carrying out the sulfiding treatment directly on metallic, ceramic or other supports which are capable of withstanding the temperature conditions involved and have molybdenum or molybdenum-containing coatings or parts that are able to participate in the reaction as reactants.

Metal surfaces on which molybdenum disulfide is to be formed in situ can be pretreated or formed in a variety of ways. Thus, for example, molybdenum or a molybdenum compound can be applied to form dense or porous but firmly adhering layers on metal surfaces by known electrolytic means, by decomposition of molybdenum hexacarbonyl on treated surfaces, by hot spraying of metallic molybdenum and by other known processes. Metal parts containing molybdenum may also be prepared by powder metallurgical processes in which metal powders, including molybdenum, are compressed into the desired shape and sintered. The metal parts so molybdenized are pretreated, if desired, by mechanical means such as sand blasting or by chemical means such as etching with dilute acids, and then sulfided expeditiously in accordance with the method of the invention to form a firmly adherent coating of crystallized molybdenum disulfide having excellent lubricating properties.

The sulfiding in situ can be accomplished by such means as heating the molybdenized surface while in contact with a molybdenum-free, alkaline bath of molten sulfur or a melt of alkali metal polysulfides.

The heating may be accomplished by any known means including local surface heating such as inductive heating and, if desired, may be combined with a preselected heat treatment of the member as a whole.

After the formation of the molybdenum disulfide, as

such or in situ, on a surface, volatile reaction by-products can be removed by further heating while reducing the pressure. In addition, undesired reaction products can be extracted with water or aqueous solutions, particularly solutions of inorganic acids such as hydrochloric, sulfuric-, phosphoric acid, and the like. Also suitable for removal of any undesired Dy-products are aqueous and aqueous-alcoholic solutions of surface-active substances, particularly cation-active substances such as organic ammonium salts and the like. Unreacted sulfur can be extracted with solvents for sulfur, particularly carbon disulfide. Furthermore, such surfaces can be after-treated, with advantage, by non-cutting shaping methods such as lapping, burnishing, and the like.

The advantages and utility of the invention will become further apparent from the detailed description in the following examples included in this description to illustrate the best modes now contemplated for practicing the invention.

Example I A mixture of one mol of M two mols sulfur and one-half mol potassium carbonate was heated to 475 C. in a closed crucible, a sample being taken every hour. After the mixture had been heated for a total of three hours at said temperature, most of the molybdenum was found to be present in the form of molybdenum disulfide. After washing with water, followed by extraction with carbon disulfide, a gray lamellar powder was obtained which had an appearance and properties very similar to those of natural, purified molybdenite.

Example 2 One mol of ammonium molybdate was heated for seven hours at 350 C. with 2.5 mols sulfur and one mol calcium hydroxide. A light gray sintered product was obtained. This was extracted with dilute hydrochloric acid until calcium ions could no longer be detected in the extract. The product was then treated with an 0.5% aqueous solution of a cation-active oxazoline derivative available on the market under the tradename Alkaterge. After this treatment, the product was found, by chemical analysis, to contain molybdenum and sulfur in a molar ratio of 1:2. The product was predominantly of a crystalline structure and exhibited a lubricating effectiveness which corresponds to that of natural molybdenite.

Example 3 A mixture of one-half mol powdered molybdenum and one mol of sulfur was heated with one mol of sodium tetrasulfide at 350 C. in a closed vessel provided with a pressure relief valve set at one-half atmosphere gauge pressure and capable of preventing penetration of air into the vessel. After cooling, it was found that about one-half of the molybdenum and sulfur used had separated out on the surface of the melt in the form of large lamellar crystals. These crystals were carefully freed from the melt, washed with water and carefully dried. The crystals were then ground to a powder and a powder pattern was determined by X-ray methods. This pattern corresponded exactly to that of natural molybdenite and tests showed the powder to have the same lubricating effectivness as natural molybdenite.

Example 4 One mol M00 2.3 "mols sulfur and 1.15 mols ammonium carbonate were heated for six hours at 450 C. in a closed, pressure-resistant iron crucible. At the end of this time, the pressure in the crucible was reduced to atmospheric and excess sulfur and decomposition products of the ammonium carbonate were allowed to escape. The vent valve was then closed again and the mixture allowed to cool- The product obtained was a molybdenum disulfide that was light gray in color and completely crystalline. A portion of this molybdenum disulfide was ground for three hours in a ball mill with a mineral oil and compared on the Four-Ball Apparatus with a similar amount of commercial molybdenum disulfide, likewise ground for three hours in a ball mill with mineral oil. It was found that with both samples the seizure load was kg. higher compared with the value obtained with an untreated oil.

Example 5 One mol M00 was melted with a one-half mol of NaOH in a crucible provided with a fritted insert through which a stream of dry hydrogen sulfide was conducted at 375 C. The melt slowly became viscous and finally sintered, forming a light gray cake. This cake, after the stream of hydrogen sulfide was cut ofi and the crucible was completely closed, was heated for two additional hours at 425 C. After cooling and washing with dilute sulfuric acid containing 0.5% of sodium dodecylbenzene sulfonate dissolved therein, the product was carefully dried. The light gray M08 powder thus obtained had. a soft feel and was ground for seven hours with one-half its weight of polyethylene glycol having an average molecular weight of about 1200 in a high speed vibrating:

Example 6 A mixture of one-half mol molybdic acid, 1.4 mols sulfur and one-half mol potassium carbonate was sintered for nine hours at 415 C. in a completely air-tight vessel capable of withstanding a pressure of three atmospheres gauge. The product obtained was washed with sulfurous acid and water, carefully dried and ground dry to a powder. It was found to contain molybdenum and sulfur in a molar ratio of 122.3.

To samples of a viscous mineral oil having a seizure load of 130 to kg. on the Four-Ball Apparatus, there were added (A) 3.2% by weight of carefully purified MoS obtained by extracting the product of this example four times with carbon disulfide, and (B) 3.4% by weight of the product extracted only with water and carefully dried. It was found that additive A had doubled the seizure load to 260 to 280 kg. and that additive B increased it even more'to 360 to 380 kg. This indicated that in some instances the extraction or purification of M08 may be omitted with advantage.

Example 7 Four Wieland bearings, described as follows, were subjected to comparative examination on an Almen-Wieland testing machine:

(a) An ordinary, untreated bearing;

(b) A bearing having a porous layer of molybdenum and mixed molybdenum oxides deposited theron elec trolytically from a strongly acidified ammonium molybdate solution;

(c) A bearing having a porous layer of molybdenum and mixed molybdenum oxides deposited thereon electrolytically as in (b) and thereupon sufided by treatment for three hours at 370 C. in a melt of sodium and potassium polysulfides. After this treatment, excess sulfiding agent adhering to the surface was removed as completely as possible by washing with water; and

(d) A bearing provided with a molybdenum disulfide coating by immersion in an alcoholic suspension of finely ground molybdenite.

The results of these tests, in which the base lubricant was the same in all instances and the load was increased in increments of 50 kg. per 100 revolutions, were as follows:

Bearing: Load capacity, kg.

(a) 250. (b) 400 (noisy operation). (c) 1500 (plus 300 revolutions). (d) 900.

Example 8 A sintered bearing containing a total of 4.3% metallic molybdenum was machined to 0.1 mm. oversize and roughened on its surface by sand blasting. The bearing was then heated for three hours at 420 C. in a melt of two parts by weight sulfur and 0.7 parts by weight of an equimolar mixture of potassium and sodium carbonate. After this treatment and careful removal by washing of all water-soluble compounds, analysis confirmed the formation and presence of molybdenum disulfide and iron disulfide on the surface. Comparison of the friction values of this bearing upon running in with those of customary bearings of the same type showed the friction of the treated bearing to be 27% less.

Example 9 The two portions of an engine shaft resting in the two main bearings were coated with molybdenum by means of thermal decomposition of molybdenum hexacarbonyl. Thereupon the entire engine shaft was dipped into a melt of a mixture of sodium and potassium triand tetrasulfide having a melting point below 300 C. At the two places where the shaft is supported in the engine and Example 10 Annular springs of ordinary dimensions were carefully cleaned in known manner by sand blasting, pickling and the like. Thin layers of molybdenum were thereupon applied to the cleaned surfaces by the hot spray process.

The springs were treated at 400 C. in a melt of sodium and potassium polysulfide containing 10% by weight additional sulfur. After 2 /2 hours of treatment, the springs were found to have thin layers of molybdenum disulfide on the surface. They were polished and found to have a friction value reduced by 23%.

It is to be expected that various modifications will occur to those skilled in the art upon reading this description. All such modifications are intended to be included within the scope of the invention as defined in the accompanying claims.

I claim:

1. Method which comprises treating a molybdenumcontaining bearing surface with a melt of sodium and potassium polysulfides at a temperature between about 300 and about 500 C.

2. Method which comprises treating a molybdenumcontaining bearing surface with sulfur in the presence of a member selected from the group consisting of potassium and sodium carbonates at a temperature between about 300 and about 500 C.

3. A structural member having a molybdenum disulfide-containing bearing surface prepared in situ by the method defined in claim 4.

4. Method which comprises sulfiding a molybdenumcontaining bearing surface which comprises contacting the bearing surface, at a temperature between about 300 and about 500 C. and in an alkaline medium selected from the group consisting of the hydroxides, sulfides and carbonates of the alkali metals, alkaline earth metals and ammonia and mixtures thereof, with a sulfidizing agent selected from the group consisting of sulfur and a compound containing sulfur in divalent form.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,843 Ruben Apr. 30, 1940 2,420,886 Lafioon May 20, 1947 2,588,234 Henricks Mar. 4, 1952 2,591,777 Bowen Apr. 8, 1952 2,707,159 Fou cry et a1. Apr. 26, 1955 2,770,527 Alderson et al. Nov. 13, 1956 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chem, vol. 11, pages 640 and 641, published 1931. 

4. METHOD WHICH COMPRISES SUFIDING A MOLYBDENUMCONTAINING BEARING SURFACE WHICH COMPRISES CONTACTING THE BEARING SURFACE, AT A TEMPERATURE ABOUT 300 AND ABOUT 500* C. AND IN AN ALKALINE MEDIUM SELECTED FROM THE GROUP CONSISTING OF THE HYDROXIDES, SULFIDES AND CARBONATES OF THE ALKALU METALS, ALKALINE EARTH METALS AND AMMONIA AND MIXTURE THEREOF, WITH A SULFIDIZING AGENT SELECTED FROM THE GROUP CONSISTING OF SULFUR AND A COMPOUND CONTAINING SULFUR IN DIVALENT FORM. 